1. Field of the Invention
2. Discussion of the Prior Art
The invention relates to an axial piston machine with RPM-dependent pressure acting against a cylinder down of the machine.
An axial piston machine according to the current state-of-the-art is known from DE 195 22 168 A1, for example. The axial piston machine disclosed in the latter consists of a drive shaft, which is mounted in a casing so as to rotate about a drive shaft axis, a cylinder barrel, which is non-rotatably connected to the drive shaft and in which cylinders are formed to accommodate axially mobile pistons, and a control plate with control ports for cyclically connecting the cylinders to a high- and a low-pressure line. Also provided is a contact pressure device for pressing the cylinder barrel against the control plate and thus preloading it against the control plate. This preloading of the rotating cylinder barrel with respect to the stationary control plate is required in order to guarantee a tight seal between the cylinder barrel and the control plate and counteract any lifting of the cylinder barrel off the control plate at high speeds. A particular requirement is to exclude the possibility of the cylinder barrel running off-center at high speeds.
The contact pressure device known from DE 195 22 168 A1 consists substantially of a contact pressure spring which is provided in the cavity between the drive shaft and the cylinder barrel and is supported at one end at the drive shaft and at the other end at the cylinder barrel, so that the cylinder barrel is preloaded with respect to the connecting block, at which the drive shaft is mounted and in which the control ports are provided. However the contact pressure spring exerts a constant contact pressure force, which is independent of the speed, on the cylinder barrel. This is detrimental insofar as the contact pressure force required is predetermined by the inertia forces which are exerted by the pistons and which increase with the square of the cylinder barrel operating speed. The contact pressure force which is exerted by the contact pressure spring must therefore be defined for the maximum cylinder barrel speed and its magnitude calculated accordingly. However this inevitably results in the contact pressure force which is exerted by the contact pressure spring operating in the same way at low speeds as well. This leads to mechanical friction losses and increased wear of the sliding partners consisting of the cylinder barrel and the control plate. An increase in the maximum operating speed must be accompanied by an increase in the spring preload exerted by the contact pressure spring, which is only possible within certain limits.
EP 0 162 238 B1, in view of the foregoing therefore proposes distributing hydraulic auxiliary cylinders over the circumference of the cylinder barrel, the working spaces of which cylinders are connected to the cylinder bores in the main cylinders. A cylinder barrel contact pressure which is dependent on the working pressure and thus the speed is achieved by means of the auxiliary cylinder. However this solution has the disadvantage of a relatively high expenditure for forming the additional hydraulic cylinders, which leads to relatively high manufacturing costs. The construction space required for the axial piston machine is also increased.
A further proposal, according to DE 195 22 168 A1, lies in providing a throttled connection between the overflow space of the casing and the overflow oil drain line in order that an additional contact pressure can be increased with the speed. The back pressure arising as a result in the overflow oil space of the axial piston machine produces an additional small axial force component by means of which the cylinder barrel is pushed in the direction of the connecting block. However this additional force component is comparatively small, as the casing wall of a conventional axial piston machine only resists a relatively low internal pressure. There is also the problem of churning losses or turbulence losses occurring at high overflow oil levels when the drive mechanism enters the overflow oil.
An additional publication to be mentioned is DE-OS 24 46 535, from which it is known to use a centrifugal force device to act on the holding-down appliance to press the slide shoes onto the oblique plate. For this purpose centrifugal weights are disposed at the circumference of the cylinder barrel and act on the pull-back ball of the holding-down appliance via a linkage and a contact pressure plate. However this centrifugal force appliance only serves to press the slide shoes against the oblique plate of the axial piston machine, which requires substantially smaller forces when compared with those required for pressing the cylinder barrel against the control plate. Moreover, the centrifugal force appliance is of a relatively low efficiency, as the linkage passing through the cylinder barrel is inclined in the radial direction and only a relatively small axial force component is therefore transmitted to the holding-down appliance. The additional construction elements in the form of the linkage and the contact pressure plate make the construction relatively complex and cost-intensive. A further undesirable feature is the increased construction space of the axial piston machine due to the arrangement of the centrifugal weights at the outer diameter. Moreover, the play in the centrifugal weight assembly is not compensated by the surrounding auxiliary or pressure elements. There is thus no guarantee that the centrifugal weights will bear against the support or pressure elements and therefore act on the appliance at relatively low speeds and when accelerating from a standstill. The result is inadequate slide shoe contact pressure in the low speed range.
It is known from DE-PS 1 226 418 to provide a centrifugal force appliance provided with a lever arm and likewise comprising centrifugal weights at the outer diameter of the cylinder barrel to press the slide shoes against the oblique plate. This appliance also has a very complex system for introducing forces. The force ranges available for pressing the cylinder barrel against the control plate in accordance with the speed are entirely different from those in the case of holding-down appliances which serve to press the slide shoes onto the oblique plate. The centrifugal force appliances known from the above publications are therefore in no way suitable for solving the problem on which the invention is based.